In a corrugator in which a corrugating medium is allowed to have a predetermined corrugation, and wherein the medium is banded to a liner so as to form a single-faced corrugated board sheet, followed by additionally bonding the single-faced corrugated board with another liner to form a double-faced corrugated board sheet, there is additionally included a group of machines for performing a subsequent series of operations upon the double-faced corrugated board sheet or double wall corrugated board sheet, such as, for example, slitting, scoring and cutting the same into predetermined length. The series of machines including the corrugator and the various processing machines as a whole constitute a so-called "corrugator line", and in connection with current technology corrugator lines, more and more emphasis is placed upon the comprehensive control of the group of processing machines within the line so as to realize rationalized production control.
In an initial stage of a current technology computer-aided corrugated board production system, a central control system (CCS) has been employed, in which a main computer control unit is designed to perform data transmitting and receiving, in a parallel manner, to and from the group of various machines within the line, and the respective machines are synchronously controlled based on such data. Within such a central control system, however, a tremendous amount of wiring is required for transmitting various data and control signals in a parallel manner to the group of various machines within the line, so that an especially elongated line of equipment, such as, for example, a corrugator suffers problems in that the number of wirings is increased and the production cost is accordingly elevated.
In order to overcome such problems, there is a tendency to adopt a decentralized control system in accordance with current corrugator line technology wherein in addition to one main control unit, a local control unit is attached to each one of the plurality of divided units of the production process, and the main control unit and the local control units are linked together within a multiplex transmission network. In this system, the synchronized production control of the entire line is achieved by means of the main control unit, while control of the individual operations within each processing unit is achieved by means of the local control unit.
Such a system can generally be called "a decentralized control systems for the corrugated line", which system has primarily been adopted for the reasons given below.
1 Users need multi-kind, small-production lots, and manufacturers have to accommodate frequent changes in the users' orders with respect to both product kind and quantity. Therefore the following control functions have become necessary under the circumstances wherein a plurality of different orders and lots are to be produced with a single corrugator line.
An operation or control function to prevent unexpected difficulties and unnecessary variations in the production speed. PA1 An operation or control function to reduce the erroneous sheet length due to order/lot changes to zero or to a predetermined minimum possible length, and to make advance production corrections so as to eliminate any excess or shortage of the products in the end. PA1 An operation or control function to synchronize the production speed between single facer and double facer machinery so as to prevent any of excessive or shortage of volume of sheets upon the bridge, and to expand the numerical data for controlling such operations. PA1 Information concerning the remaining sheet length at each machine so as to improve the accuracy of the sheet splice position. PA1 Information concerning the variable lengths of the rolling capacity of each preheater for the cardboard webs to be fed, the stay-on-the-line volume, and the like. PA1 Numerical information concerning the cardboard web stop/movement characteristics during splicing. PA1 Information concerning each machine condition so as to determine the optimum production speed. PA1 (1) Influences of the processing time including that of transmission within the decentralized control system, that is errors in the feed length, can be reduced to almost zero. PA1 (2) The control function program including the transmission among decentralized local control units can be planned without the need for considering the processing time. PA1 (3) Since the production feed length positions determined so as to have an absolute value based upon the feed length reference and production reference data, the control program can be commonly used by means of all control units. PA1 (4) If the real-time actual control file is prepared by including the production reference values in the key codes of the production data based upon which the production history is written and the operational results are aggregated, both the file and the processing program can be simplified .
2 In response to the need for more operative functions, higher operational speed, and higher accuracy of the corrugator line, the control volume and high-speed response within each processing unit and the machines operatively associated therewith must be increased, and furthermore the following various information concerning other processing units, including the production command data, must be incorporated and correlated.
3 To reduce the number of defective products and thereby reduce costs and resources, each processing unit of such a current technology corrugator line has been rendered extremely high performance and the detected data has increased in volume following the progress in the product quality control by means of computers and the tracking control of deteriorated quality parts. This data must be commonly used by other processing units.
4 With the improvement of machine performance, the need to save labor at the work site has increased. Monitoring equipment and operation by means of knowledgable operators have become necessary in order to guarantee the safe and efficient operations, such functions and operations requiring the supply of more appropriate information and more diversification of the information contents.
For instance, a large-sized multi-purpose display board and further intelligent monitoring devices are needed.
5 Standardization, high maintainability and improvement in loss preventive ability of the production control system are required.
While the aforenoted decentralized control system is extremely useful and can satisfy the diverse users' and manufacturers' needs, it is confronted with new problems to be solved, which are related to the real time performance required when the decentralized unit-process control systems within the corrugator line are connected with a multiplex transmission network so as to realize the control functions by means of a computer program.
That is to say, the decentralized control system for a corrugator line is required to perform the functions of first transmitting the production plan instruction data, performing real-time communication between the control systems of processing units concerning the production conditions which change with the passage of time, and finally collecting the data for production results for the purpose of achieving the rationalized production control in connection with the production of corrugated board sheets. Thus, when these real-time communications are generated between the unit-process control systems, there is the problem of how to deal with the communication processing time which significantly affects the final performance of the system.
For example, the transmission of the production control information must satisfy the required performance of the control system and an ideal transmission function for the synchronism of the operations which can occur simultaneously in connection with the positional relationships defined between the plurality of unit-process control systems such that the same are suitable for this requirement to be realized. However, when the 2.5 Mbps performance is used within the present practical local area network (LAN) capable of N:N communications, its response is approximately 5 msec. (millisecond) at best, which value further increases by one figure when it is used within a large system with additional nodes (that is, the number of links within the multiplex transmission network). The "deviation error" of the position upon the production sheet due to the processing time, and when combined with other error factors, causes dissatisfaction of the specification requirements, so it was therefore desired to reduce the control errors as close to zero as possible. However, the computer system using an ideal highspeed local area network (LAN) producing no errors due to the processing time is extremely expensive at the present time.
If the production speed of a corrugator line is assumed to be, for instance, 360 m/min., the deviation error will be 30 mm or more (360 m/min..times.1/60 min/sec..times.0.005 sec..times.1000 mm/m=30 mm) when considering the aforenoted response performance.
On the other hand, the production control information communications generated between the node controllers within a decentralized control system is not only monodirectional, but may be bi-directional. For instance, when the node A controller requests the node B controller to inform it of the remaining sheet length in order to carry out the same positional operation, the latter controller extracts the remaining sheet length information and transmits its data to the node A controller. By means of such information, the node A controller can determine the required position of the production sheet, but in view of the fact that such has been derived as a result of bi-directional information interchange, twice the errors will occur per communication of each factor.
In addition, when these operational results are communicated to the node C controller so as to further adjust the process C to the same position, further errors will, needless to say, be added thereto. In other words, within the line control system wherein this repetition processing comprises additional steps, the deviation error increases as the steps increase in number. Thus this error varies so much according to the system size and control volume that it cannot be quantified (this is the reason why it is simply called "the deviation error").